Generally, separation of a water-organic solvent has been performed by pervaporation, and much reserches has been devoted to the method. Separation of water-organic solvent can also be carried out by other than pervaporation, for example, by reverse osmosis or low pressure reverse osmosis. Several studies on this matter have been carried out.
Component materials of membranes for the separation of water-organic solvent which have been developed until now can be largely be divided into two types: Hydrophilic materials such as polyvinyl alcohol (J. of Memb. Sci, 51(1990) 273-292, J. Memb. Sci., 51 (1990) 215-226), polyacrylic acid (J. Appl. Polym. Sci., 41 (1990) 2133-2145) and cellulose (J. Appl. Polym. Sci., 40 (1990) 633-643, J. Appl. Polym. Sci., 37 (1989) 3385-3398), and hydrophobic materials such as polysiloxane (J. of Memb. Sci., 49(1990) 171-205). Hydrophilic materials are useful for purifying organic solvent, and therefore have been widely used for the production of membrane which can be used to separate and remove minor amount of water in alcohols. The development of the membrane for the separation of water-alcohol using the above mentioned materials has made much progress up to now.
Hydrophobic materials, contrary to the hydrophilic materials can be used for the production of separation membrane to separate and remove minor amount of organic solvent in water, i.e. alcohols or chloro compounds in water. Much research in this field is also being actively carried out.
In the case of removing minor amount of water in organic solvent, the membrane used for this purpose should have good hydrophilicity. This is because if the hydrophilicity is good, water in organic solvent is selectively absorbed into membrane and then the absorbed water selectively pass through membrane, and thereby selectivity and permeability of membrane can be acquired to be a good membrane for separation. At present numerous types of polymers having good selectivity to water are known. However, among them, very few polymers can be used for the production of membrane for separation. Polymers which have been most frequently used are poly(vinyl alcohol), poly(acrylic acid) and cellulose type polymers. Those materials are polar polymers which reveal good affinity to water. For this reason, many studies of membrane using those materials have been carried out. However, when hydrophilicity of those materials is compared with that of ionomer in which ionic group is main chain or side chain of polymer, it is clear that ionomer has much higher hydrophilicity. Poly(vinyl alcohol), poly(acrylic acid) and cellulose type polymers have restricted affinity to water since they do not have ionic group.
In the case of separating water selectively from mixtures containing small amounts of water, hydrophilicity of membrane is the most crucial factor with respect to capacity of the membrane. In particular, hydrophilicity of the membrane is indispensable to the selectivity of the membrane.
To avoid the above mentioned problems, new materials in which an ionic group was introduced to main chain or side chain of the polymer appeared. Most of them are produced by treating poly(acrylic acid) with a base such as sodium hydroxide, and some are produced by treating cellulose with chloride. These materials have good affinity to water and therefore the membranes produced from these materials have fairly good quality.
However, these materials have deficiencies in that stability of the membrane is decreased since ionic groups in the materials may be neutralized by the action of acidic materials contained in the water or mixture. Hydrophilicity of the membrane decreases as ionic character of the membrane disappears. Decrease of selectivity of membrane also naturally ensues from the decrease of affinity to water. Therefore, the quality of the membrane decreases greatly as time passes although the membrane has excellent separation capability at first.
To overcome the above mentioned problems, Maeda et at. suggested an ion complex membrane made by using polyacrylic acid in Journal of Applied Polymer Science, Vol.42, 3229(1991). However, the production of ion complex membrane by using polyacrylic acid according to Maeda et al. is troublesome since polyacrylic acid have to be made in the anion state to be used in the production of an ion complex membrane. Therefore the ion complex membrane suggested by Maeda et al. still leaves much to be desired.
Given these considerations, it is clear that any ion complex separation membrane which would have good separation capability to separate minor amounts of water in an organic solvent and good stability would have significant advantages over the prior art membranes.
Based on the result of the above-mentioned studies, the present inventors have made extensive studies in order to develop a membrane for separation which is useful for water-organic solvent separation by pervaporation and as well as by other method, i.e. reverse osmosis or low pressure reverse osmosis. As a result, the present inventors have now found that by preparing cationic polymer, and then dipping the cationic polymer in an anionic polymer solution, an ion complex membrane can be produced. Based on these findings, we have completed the present invention.